This invention relates to a process for the preparation of alkali metal salts of 2-phosphonobutane-1,2,4-tricarboxylic acid by alkaline saponification of the corresponding 2-phosphono-1,2,4-tricarboxylic acid pentaalkyl esters or partial esters.
It is known that phosphonic acids and their soluble salts are suitable for the sequestration of alkaline earth metal ions in aqueous systems. The following compounds are examples: Nitrilotrimethylene phosphonic acid, 1-hydroxyethane-1,1-diphosphonic acid, 1-phosphonopropane-1,2,3-tricarboxylic acid, phosphonosuccinic acid and 2-phosphonobutane-1,2,4-tricarboxylic acid (DE-A-20 61 838, EP-A-59 897).
Phosphonic acids manifest the so-called threshold effect, i.e. the formation of an insoluble precipitate of calcium carbonate in water containing lime is prevented even by the addition of one of these compounds in less than the stoichiometric quantity, based on the lime content; hence also the term "threshold inhibitors".
The possibility of commercial production of the above-mentioned phosphonic acids has been of economical interest for some time. Since some of these compounds, e.g. 1-phosphonopropane-1,2,3-tricarboxylic acid, phosphonosuccinic acid or 2-phosphonobutane-1,2,4-tricarboxylic acid, are prepared from their esters by saponification, there is also great interest in a process for converting the esters into the free acids or their alkali metal salts. The alkali metal salts are always preferred to the free acids when the threshold inhibitors are to be used in alkaline solutions or stable, storable powders are to be produced by drying.
Numerous experiments have therefore been carried out in recent times with a view to improving the conventional process for the preparation of phosphonic acids, from which the corresponding alkali metal salts are subsequently obtained by neutralisation with alkali metal hydroxide solutions, in which process the corresponding esters, in particular the methyl esters, are hydrolysed in the presence of catalytically active mineral acids or in the presence of hydrogen halide.
An important advance was achieved by the process described in DE-A-2 229 087, in which saponification is carried out at temperatures from 90.degree. to 150.degree. C. without the addition of foreign acids and the phosphonic acid to be produced is itself used as catalyst. The corresponding alkali metal salts are then obtained by neutralisation with alkali metal hydroxide solution after complete saponification. Disadvantages of this process are found to be the long reaction time required for complete hydrolysis, the necessity of removing the heat of neutralisation produced and, in the special case of 2-phosphonobutane-1,2,4-tricarboxylic acid, which is technically particularly important, the formation of sparingly soluble alkali metal salts of a partially neutralised 2-phosphonobutane-1,2,4-tricarboxylic acid. Other advantageous further developments of this process without the addition of foreign acid are described in DE-A-2 441 783, DE-A-2 441 878 and DE-A-2 745 982.
The object therefore arose of providing a process which could be carried out both batchwise and continuously, in which the above-described disadvantages in the preparation of soluble alkali metal salts of 2-phosphonobutane-1,2,4-tricarboxylic acid are avoided.